1. Field of the Invention
The present invention relates to the field of optical disc apparatuses, and more particularly, to an optical disc apparatus capable of reproducing a wobble signal from an optical disc manufactured by wobble modulation and to a method of reproducing a wobble signal.
2. Description of the Related Art
Recordable optical discs, such as CD-R/RW, DVD-RAM, DVD-R/RW or DVD+RW optical discs, have guide grooves formed along tracks in order to help an optical pickup to follow a desired track exactly. The guide grooves are referred to as grooves, and the remaining portions are referred to as lands. Recordable discs are classified into optical discs to record user data on only either of lands or grooves, such as CD-R/RW, DVD-R/RW or DVD+RW optical discs, and optical discs to record user data on both lands and grooves, such as DVD-RAM optical discs.
Upon manufacturing a recordable optical disc, a wobble signal to control the rotating speed of an optical disc and to obtain synchronous information is recorded on the groove tracks and/or land tracks of the recordable optical disc by changing the amplitude direction of the wobble signal.
Physical identification data (PID), which is sector address information, is also recorded upon manufacturing an optical disc, in order to recognize the physical locations on the optical disc. Recording address information on uneven pre-pits formed in the header area reduces the capacity of an optical disc. In addition, the above-described address information recording technique is bad in the aspect of compatibility with reproduction-dedicated optical discs. Thus, a technique of recording address information by modulating the frequency/phase/amplitude of a wobble has been proposed, which means recording of address information by adding a periodic change in frequency, phase or amplitude to a wobble signal with a single frequency.
As is well known in the technical field of the present invention, CD-R/RW optical discs record address information by frequency modulation in which the period of a wobble is modulated, whereas DVD+RW optical discs record address information by modulating the phase of a wobble. As CD-R/RW and DVD+RW optical discs record user data only on grooves, they generally can record address information on a frequency-modulated or phase-modulated wobble formed on both sidewalls of each groove track.
The restoration of address information recorded by addition to a wobble signal as described above uses a wobble signal reproduced from an optical disc.
FIG. 1 is a top view of a part of an optical disc where bi-phase shift keying (BPSK) wobble by an address-in pregroove (ADIP) manner used to format a DVD+RW optical disc, that is, bi-phase modulated wobble, is formed.
In the related art, a radial push-pull signal (tracking error signal) detected by a photodetector of an optical pickup is used as a wobble signal to restore address information on an optical disc on which wobble is formed, as illustrated in FIG. 1. If address information has been recorded on an optical disc by modulating the phase of a wobble, it is restored by extracting a wobble signal with a specific frequency from a radial push-pull signal and by demodulating the phase of the wobble signal. Accordingly, accurate reproduction of a wobble signal recorded on an optical disc is significantly important to properly restore address information. Here, a radial push-pull signal is obtained by performing a subtraction on electric signals detected from two parts into which a light beam reflected by an optical disc is divided in a direction corresponding to the radial direction of the optical disc.
However, in a conventional wobble signal reproduction manner using a radial push-pull signal, if the track pitch is narrowed to increase the recording density of an optical disc, an interference (cross-talk) due to wobbles of adjacent tracks degrades the signal-to-noise (S/N) ratio of a reproduced wobble signal. This hinders accurate restoration of address information.